The present invention relates to a process for improving the stability of acetal polymers of converting the unstable terminal hydroxyl groups of said polymers into groups of higher stability, and more particularly into ester or ether groups by treatment with orthoesters, orthocarbonates or anhydrides of organic acids. The acetal polymers which may be treated by the process of the present invention are those having molecular weights greater than about 10,000, which are prepared in the art by the polymerisation of an aldehyde or the copolymerization of one or more aldehydes with non-aldehyde monomers. Those acetal polymers which are obtained from cyclic oligomers of aldehydes, such as, for example, trioxan and tetraoxan (oligomers of formaldehyde) by homo-polymerization or by copolymerization with non-aldehyde monomers may also be treated by the process of the present invention.
It is known that acetal polymers resulting from polymerization and copolymerization processes contain at least one terminal hydroxyl group per macromolecule.
The unstability of these hydroxyl groups brings about a gradual degradation of the macromolecular chains at the temperatures required for the processing of the polymers. It is therefore necessary to treat the latter in order to transform the hydroxyl groups into other groups of higher characteristics of thermal and chemical stability. In particular these characteristics of stability may be conferred by transforming the hydroxyl groups into ester groups by reaction with anhydrides of carboxylix acids, especially acetic, propionic and butyric acids, or into ether groups by reaction with orthoesters, dialkylacetals, ketals and organic orthocarbonates.
The known processes may be carried out by different methods and in practice there are four basic techniques which may be summarized thus:
the end-capping agent (esterifying or etherifying agent) is brought into contact with the acetal polymer dispersed in a liquid medium which is not a solvent for the polymer; this liquid medium may consist in part, or wholly, of the end-capping agent.
the end-capping agent is brought into contact with the acetal polymer dissolved in a liquid which is a solvent for the polymer; this solvent liquid may consist wholly or in part of the end-capping agent.
the end-capping agent is brought into contact with the acetal polymer dispersed in a mixture of two liquids consisting of a solvent and a non-solvent for the polymer, these liquids being completely miscible at the treatment temperature.
the end-capping agent in the vapour state is brought into contact with the acetal polymer in the form of fluidized solid particles.
Techniques of this type are, for example, described in U.S. Pat. Nos. 2,998,409, 2,964,500, 3,192,182 and 3,875,117 and in British Pat. No. 880,737.
The processes described are generally satisfactory in the sense that an end-capped (esterified or etherified) product is obtained, although differences, which may even be considerable, occur in terms of the economy of the processes and the yield and properties of the end-capped polymers. One common characteristic of the processes under discussion is that they need relatively high quantities of the end-capping agent and/or the solvent or dispersing liquid compared with the quantity of polymer subjected to the end-capping treatment. In particular, in processes in which the reaction is carried out in the liquid phase, the weight ratio between the liquid and the acetal polymer is at least 3:1 and may be up to 15:1.
In the cases cited, specific technological and also chemical problems occur depending on the particular system chosen for the reaction. Thus, for example, when the end-capping reaction is carried out by the solution method, the precipitation of the end-capped acetal polymer is a point of considerable difficulty. On the other hand, when the reaction is carried out by the fluid-bed method, high quantities of gas and/or vapour are required and there are further difficulties in achieving intimate contact between the solid particles of the polymer and the end-capping agent. Moreover, in all the cases cited, the presence of large quantities of liquids and of end-capping agents makes the process difficult to carry out economically due to the need to recover both the unreacted end-capping agent and the liquid used for dissolving or dispersing the polymer as completely as possible.
A need is also felt for carrying out the reaction with small quantities of end-capping agent in view of the ease with which this agent interacts with the aldehyde monomer which is formed due to degradation of the acetal polymer. Thus, for example, in the case of the esterification of polyformaldehyde, the formaldehyde monomer which is formed reacts with acetic anhydride and thus forms by-products of the methylene-glycolacetate type which are difficult to separate and harmful to the acetal polymer, above all in terms of the colour of the polymer.